There are generally two types of surface elastic wave devices. The first type is referred to as surface acoustic wave (SAW) devices. SAW devices may function as filters, oscillators or resonators. The second type is referred to as surface skimming bulk wave (SSBW) or shallow bulk acoustic wave (SBAW) devices.
In general, elastic waves are categorized into three modes of vibration--longitudinal (compressional), vertical (shear), and horizontal or transverse (shear). In surface acoustic waves, which are also referred to as Rayleigh waves, the longitudinal and vertical (shear) modes are present. In surface skimming bulk waves, only the horizontal (shear) mode is present. Examples of devices utilizing SAW or SSBW include those disclosed and illustrated in P. Cross and S. Elliott, "Surface-Acoustic-Wave Resonators," 32 Hewlett-Packard J. 9 (Dec., 1981); and T. Lukaszek and A. Ballato, "Microwave Frequency Sources Employing Shallow Bulk Acoustic Wave Devices," Microwave J. 77, (Mar., 1985).
As best shown in FIG. 1A, a conventional surface acoustic wave device such as a delay line, designated 12, would include a piezoelectric substrate 14 such as quartz having at least one surface 16 on which an input transducer 18 and an output transducer 20 are positioned. Substrate 14 is generally a single crystal quartz, or any other low-loss piezoelectric material, that has a diameter of approximately 5 centimeters and a thickness of approximately 0.5 millimeters. A commonly used piezoelectric material is lithium niobate. Surface 16 of substrate 14 is highly polished. Each of transducers 18 and 20 includes a plurality of parallel metallic bars or fingers, generally referred to as interdigital transducers (IDT's). The IDT's, generally manufactured from aluminum, are deposited onto surface 16 of substrate 14 by conventional semiconductor fabrication techniques. Each of the bars or fingers is spaced from an adjacent finger by half a wavelength, .lambda./2, where .lambda.=v/f. In particular, "f" is the excitation frequency, "v" is the surface wave velocity and ".lambda." is the wavelength. Each of the fingers has a thickness of approximately 500 to 5,000 Angstroms(.ANG.).
An alternating voltage of frequency "f" is then applied to the input or transmitting IDT's, creating an electric field between adjacent fingers. A stress field is then generated by the electromechanical interaction in the piezoelectric substrate immediately adjacent to the input IDT's. The vibrational waves produced by this stress field then propagate away from the IDT's as surface waves. The surface waves can be detected by a second set of remote, receiving IDT's. The detection of the surface waves is carried out by an inverse process in which the received surface waves create an electric field between the fingers of the receiving IDT's, generating a voltage that could be sensed.
If the SAW device is a resonator, it would, in addition to the IDT's, include reflectors. Each of such reflectors includes a plurality of metallic fingers which are similar to those of the transducers. The reflectors could also be manufactured as shallow grooves etched by conventional semiconductor fabrication techniques. Other periodic perturbations may also be used. In SAW resonators manufactured for use in the UHF range (30-3,000 MHz), the width of each groove, when grooves are used, varies from 0.2 to 20 microns and adjacent grooves are separated by a width of the same dimension.
The operational range of the reflector IDT's, i.e., the band of frequencies, is determined by both the periodicity and depth of the grooves when grooves are employed as reflectors. Since a surface acoustic wave generally decomposes into reflected longitudinal and vertical (shear) waves when it encounters an abrupt surface discontinuity such as the edge of the substrate, the design of the metallic bars or grooves is of paramount importance. The metallic bars or grooves must be "small" enough, i.e., either low heights for the bars or shallow depths for the grooves, such that the SAW would encounter a sufficient number of such reflectors (typically 1000) whereby a coherent reflection of the SAW would be generated at a particular frequency. Concomitantly, the reflectors cannot be so "large", i.e., great heights for the bars or great depths for the grooves, such that the SAW cannot propagate completely through the array of reflectors at that particular frequency.
SAW devices include advantages and disadvantages. Its foremost advantage is its inherent characteristic of maintaining or trapping its energy to surface 16 of substrate 14. As best shown in FIG. 1B, most of the Rayleigh wave energy is present within the topmost one wavelength, as measured from surface 16. This phenomenon is due to the fact that the two modes of Rayleigh waves, i.e., longitudinal and vertical (shear), cancel each other such that they do not diffract into the body of substrate 14, i.e., spreading away from surface 16. Thus, a SAW would propagate in the horizontal direction, maintaining its energy close to surface 16.
Its foremost disadvantage is that when high-Q (quality factor) characteristics are desired in a device such as a resonator, manufacturing difficulties are encountered. The Q factor is a comparison of the energy stored in a device in relation to the average power loss of that device at a particular frequency. This value is without a unit of measurement. If the resonator is to be used at frequencies greater than one gigahertz, the required metallization for the fingers becomes so small that semiconductor fabrication techniques have not yet been perfected to perform such a delicate deposition. In addition, the device losses due to such thin metallization, as well as the viscosity losses of the SAW in a substrate manufactured from known materials, become prohibitively large.
As best shown in FIG. 2A, a surface skimming bulk wave device, designated 30, also comprises a piezoelectric substrate 32 having at least one surface 34. An input IDT transducer 36 and an output IDT transducer 38 are provided on surface 34. SSBW device 30 illustrated in FIG. 2A is a delay line. Instead propagating Rayleigh waves, transmitting transducer 36 would launch a transverse (shear) wave that is not only parallel to surface 34 of substrate 32 but also orthogonal to the direction of propagation, as best shown in FIG. 2A.
The foremost disadvantage of SSBW devices is the inherent characteristic of the transverse (shear) waves of not propagating along surface 34 of substrate 32. Instead, the transverse (shear) waves travel at a grazing angle with respect to surface 34. If such propagation is unimpeded, the waves will eventually diffract entirely into the body of substrate 32. Having such a characteristic of propagation, the energy of the waves is not maintained within the topmost one wavelength, as best shown in FIG. 2B. As a result, the amount of energy detected by output transducer 38 is substantially smaller than that transmitted by input transducer 36. Such diffraction or scattering is especially troublesome at the discontinuities such as the interface between a free substrate surface and an IDT. Thus, the distance between transducers is a limitation in many of the applications utilizing SSBW devices.
The foremost advantage of an SSBW device is its inherent characteristic of permitting the propagating surface skimming bulk waves to travel at a higher velocity. This higher velocity characteristic, which can be as much as 60% higher than surface acoustic waves, enhances its use as filters and delay lines. Another equally important advantage is its inherent characteristic of less material attenuation, which is generally referred to as a "less lossy" condition. Material attentuation is generally defined as that portion of the energy which has been lost due to heating of the atoms and molecules of the substrate. In addition, the material attentuation of the substrate relates to the crystalline structure of the substrate. Whether or not a particular substrate is suited for either SAW or SSBW application is within the knowledge of one skilled in the art. Thus, it is well known that substrates having optimal SSBW properties, e.g., the ST-cut quartz, also minimize material attentuation. Thus, the less lossy property of SSBW devices permits the presence of more power in SSBW devices. More power may be used in SSBW devices since the depth of travel of surface skimming bulk waves is deeper than that of surface acoustic waves which travel very close to the surface of the substrate. Power is related to the fact that particle motion in the body of the substrate is greater than the particle motion at the surface of the substrate.
Other forms of losses in both SAW and SSBW devices include transduction and diffraction losses. Transduction losses, which are generally negligible, relate to the loss in energy during the conversion of electrical energy to vibrational energy, or vice versa, by the IDT transducers. Diffraction losses, which are of paramount importance in SSBW devices, relate to the diffraction of propagating waves into the body of the substrate.